Rotary furnaces are employed for continuous processes in process engineering. As a rule, a rotary furnace consists of a cylindrical rotary tube that is sometimes many meters or dozens of meters long and that has a furnace shell generally made of metal. In this context, the furnace shell is slightly inclined so that the rotation of the furnace shell causes the material to be transported inside the furnace along the axis of rotation of the furnace shell from the higher inlet side to the lower outlet side. The material that is to be processed can vary and can comprise, for instance, solids, stones, slurries or powders. The requisite processing temperature can be established directly or indirectly in the furnaces. When it comes to materials that call for a high processing temperature, the rotary furnace is heated directly, for example, by means of a lance in the form of a burner situated on the outlet side of the rotary furnace, said lance being located approximately in the middle of the rotary furnace. Directly heated rotary furnaces are used, for example, for cement production, for lime calcining, to melt ceramic glass, to melt down metals, for iron reduction, to produce activated carbon as well as for other applications. In this process, the directly heated rotary furnaces are operated at very high temperatures. During cement production, for example, the raw materials, namely, lime and clay are ground up and calcined in the rotary furnace at approximately 1450° C. to form so-called clinker and subsequently cooled off and further processed after leaving the rotary furnace.
Rotary furnaces that are exposed to such high temperatures have a furnace shell made of stainless steel or of high-temperature steel that can be exposed to temperatures of up to 550° C. or 950° C., respectively. Since the temperatures in the directly heated area are considerably higher, the inside of the furnace shell made of steel is lined with high-temperature ceramic elements. In this context, the thickness of the lining determines the temperature to which the steel shell is exposed during the process. In order to prevent the furnace shell from warping during operation due to the temperature load or in order to prevent damage to the inner lining that would cause the furnace shell to bend or even melt, nowadays the furnace shell is cooled from the outside by means of air fans that are arranged on the outside of the rotary furnace over the entire length of the furnace shell.
Such a cooling technique is complex and takes up a great deal of space around the furnace. Moreover, such a fan cooling system is very noisy and uses a lot of electricity, which is expensive. If the noise pollution of the environment has to be diminished for noise-protection reasons, the rotary furnaces would have to be operated in a soundproofed hall, which would not be advantageous because of the high processing temperatures and which would also be prohibitively expensive due to the cost of the building. Moreover, such a fan cooling system can neither detect nor individually cool strong localized hot spots on the furnace shell.
Before this backdrop, it would be desirable to have a cooling system for rotary furnaces that can be easily and reliably operated at a low noise level, that allows localized cooling control and that reduces the power consumption.